Burner safety control system



Nov. 8, 1955 c. HOTCHKISS 2,722,977

BURNER SAFETY CONTROL SYSTEM Filed Oct. 15, 1951 Z6 7 PILOT SWITCH [0THERM STAT 5 F i Z 3 2 I0 l PILOT 1 SWITCH 1 14 'i I l 6 l l g l 3 6. 5O I S TAT wi l l P /5 ,8 M 38 44 48 56) l lllb INVENTOR.

[LIFFEIRD HUTEHKIEJE ATTEIR'NEYS' BURNER SAFETY CONTROL SYSTEM CliffordHotchkiss, Milwaukee, Wis., assignor, by mesne assi nments, to GeneralControls Co., a corporation of California Application October 15, 1951,Serial No. 251,430

2 Claims. (Cl. 158-123) This invention relates to control systems andmore particularly to combined safety and condition controlling means forfuel burners and the like.

Electrically operated fuel control apparatus has been arranged to permitsupply of fuel to the main burner if a flame sensitive portion of theapparatus has previously been heated by a pilot burner. This conditionis indicative of a proper preparatory state which is required prior toignition of the main burner. The flame sensitive device may take theform of an electrical resistance element which changes its resistancewhen heated and thus may be utilized to affect the current in acontrolling circuit. This invention utilizes a type of resistanceelement that is wound with Hytemco wire, an alloy of nickel and iron,having a relatively large positive temperature coeflicient ofresistance, i. e., the resistance is a function of temperature andvaries proportionally therewith. It is this characteristic that isutilized to perform a novel switching function in this invention.

It is one of the objects of this invention to provide a novel type oftemperature responsive resistance pilot which may be used in a gasburner control system to effect operation of the main gas burner underdesirable safety conditions.

it is one of the objects of this invention to provide a novelthermostatic pilot switch and accompanying circuit components to preventoperation of the gas burner until satisfactory ignition of the pilotburner has been realized.

In essence, the invention consists of a novel thermostaticallycontrolled switch operated by a parallel circuit to effect operation ofa main gas valve. The parallel circuit includes as one of itscomponents, a temperature responsive resistor, located in proximity tothe pilot flame and responsive thereto to effect a wattage balance orunbalance in the parallel circuit, depending upon the presence orabsence of flame.

In the drawings Fig. 1 represents an across-the-line type of wiringdiagram showing the invention as applied to a gas burner control system;Fig. 2 represents a semi schematic type of wiring diagram showing thepilot and the thermostatic pilot switch in their functional relationshipto the control circuit; Fig. 3 represents a plan view of thethermostatic pilot switch; and Fig. 4 represents a right side view ofthe switch of Fig. 3.

Referring now to Fig. 1, a conventional step-down transformer 2 isshown, having its primary connected to a source of voltage through amanual switch 3 and its secondary connected to the branch lines 4 and 8.A resistance type heater6 is connected between the lines 4 and 8 to besubjected to the full voltage of the secondary of transformer 2. Oneside of a temperature responsive resistor 10 is connected to branch line4 and the other side to one side of a resistance type heater 12, similarto heater 6. The other side of heater 12 is connected to the line 8.

A thermostatic pilot switch 14 is indicated in Fig. 1 by the character14a because the heaters 6 and 12, which are a functional part of theswitch 14, are located in nited States Patent other portions of thecircuit. The switch 14a represents only the contact switch portion ofthe assembly in its relationship to the controlling circuit of Fig. 1.This may be further explained by reference to Fig. 2, in which the boxedarea is indicated as the pilot switch 14 and includes heaters 6 and 12.

In the wiring diagram of Fig. 1, one side of switch 14a is shownconnected to the line 4 and the other side to one side of a roomthermostat 16. The thermostat 16 is operated to a closed contactposition upon a demand for heat in a room or other enclosure and isoperated to an open contact position upon the satisfaction of the heatdemand. The other side of thermostat 16 connects with one side of theenergizing portion of a control device such as a main gas valve 18controlling the flow of gas to a main burner, not shown. The other sideof valve 18 is connected to the branch line 8. The valve 18 may be asolenoid valve in which the energizing portion is a solenoid coil whichactuates the valve to open position when energized and closes the valvewhen deenergized. Both the thermostat 16 and the valve 13 areconventional types of apparatus utilized in gas burner control systemsand their functions are well understood in the art.

Referring now to Fig. 2 in which is shown a semi-schematic diagram, afragmentary view of a typical gas pilot burner 20 is shown in thermalproximity to the temperature responsive resistor 10. The resistor 10 maybe enclosed in a bulb or capsule (not shown) to prevent damage from thepilot flame. The main requirement for the resistor 10 is that it beexposed to the heat fo the flame at the pilot burner. Thus, with thesuccessful ignition of the pilot 20, by any suitable ignition means, thepilot flame will impinge on the bulb containing the resistor 10 tothereby heat the resistor 10 and change its resistance. The temperatureresponsive resistor 10 has a positive temperature coefiicient ofresistance, which means that upon a temperature increase, the value ofelectrical resistance will increase and conversely upon a temperaturedecrease the value of electrical resistance will decrease. A resistorhaving a negative temperature coeflicient of resistance, such as athermistor, could be used as will be pointed out hereinafter, but forpurposes of explanation, this invention is shown utilizing a resistorhaving a positive temperature coefficient of resistance. Thus theresistor 10, for example, may have a resistance of approximately 20 ohmswhen cold (that is at ordinary room temperatures), and a resistance ofapproximately ohms when hot (that is at high temperatures occasioned bythe pilot flame).

The pilot switch 14 is illustrated schematically in Fig. 2 to show itsfunction in the control circuit, but reference is now to be had toFigures 3 and 4 in which the construction details are shown. Aninsulation block 22 provides a means for mounting or supporting theswitch components and for fastening the switch assembly to a panel, etc.by means of a screw 24. A relatively stiff strip 26 of electricalconducting material is secured to one face of the block 22 by means of ascrew 28. A similar strip 36 is secured to the opposite face of block 22by means of a screw 32. The screws 28 and 32 also serve as electricalterminals for connecting the switch 14 in the control circuit (Figs. 1and 2). Set screws 34 and 35, threaded into block 22, are provided toposition the strips 26 and 38, respectively, relative to the block 22and to each other. This provides adjustments for different ranges ofoperation and for calibration of the pilot switch 14.

The free end of strip 26 has a temperature responsive element, such as abimetallic blade 36 riveted thereto. The heater 6, previously describedas to its electrical connections in the circuit, is mounted on bimetal36 in thermal proximity thereto, as shown in Fig. 3. A contact structure38 is afiixed to the free end of bimetal 36. The

contact 38 may be moved by adjustment of screw 34 or by flexure ofbimetal 36.

A second temperature responsive element, such as a bimetallic blade 40and a co-extensive leaf spring 41, are secured to a mid point on block22 by means of a retaining strip 42 and a screw 43. The bimetal 40 issimilar in construction and characteristics to the bimetal 36 anddeflects similarly with the same amount of applied heat. The spring 41carries at its free end a contact 44, which co-acts with contact 38. Thespring 41 is biased in a downward direction against the bimetal 40 (Fig.3). An opening 46 in bimetal 40 permits upward movement of contact 44and spring 4-1, independently of bimetal 40. The heater 12, heretoforedescribed in its electrical position in the circuit, is mounted on thebimetal 48 in thermal proximity, as shown. A contact 48, similar tocontact 44, is secured to the free end of bimetal 48 and moveabletherewith.

A third temperature responsive element, such as a bimetallic blade 50and a co-extensive leaf spring 52 (similar to spring 41) are riveted tothe free end of strip 30. The free end of bimetal 50 has a semi-circularaperture 54 therein which allows a contact structure 56, attached to thefree end of spring 52, to protrude therethrough. The spring 52 andcontact 56 are biased in a downward direction against the bimetal 50.Note that downward travel of both contacts 44 and 56 is limited by thebimetals 40 and 50 respectively, and the upward travel is resisted bythe bias of springs 41 and 52, respectively.

The currentcarrying path through the switch 14 is as follows: from thesecondary of transformer 2, branch line 4, terminal 28, strip 26,bimetal 36, contact 38, contact 44, spring 41, bimetal 48, contact 28,contact 56, spring 52, strip 3%), terminal 32, thermostat 16, valve 18,branch line 8 and back to the secondary of transformer 2. Thus theswitch contacts are in series relationship and separation of eithercontacts 38-44 or contacts i8-56 will interrupt the switch circuit. Asshown in Figs. 2, 3, and 4, the switch 14 is in open-circuit position,because contacts 48-56 are separated even though contacts 33--44 areclosed. This is the cold position of the switch, that is, when nocurrent is flowing in the circuit.

in the Wiring diagram shown in Figs. 1 and 2, three branch parallelcircuits are utilized to control the gas supply and demand system. Theheater 6 with its connections to branch lines 4 and 8 forms a firstparallel circuit. The temperature responsive resistor 10 and heater 12with their connections to branch lines 4 and 3 forms a second parallelcircuit. The first and second parallel circuits comprise a parallelsystem which is designated as the controlling system. A third parallelcircuit is formed by the switch 14a, thermostat 16, gas valve 18 andtheir connections to branch lines 4 and 8. The third parallel circuit isthe circuit controlled or the demand circuit because it contains thethermostat 16 which responds to a heat demand and it contains the pilotswitch 14a which is controlled by the parallel system, above mentioned.

For the purposes of illustration, the following is a description of theparallel system components and their resistance values under balancedand unbalanced conditions. The heater 6 has a fixed resistance of 360ohms under all conditions of operation. The heater 12 has a fixedresistance of 90 ohms under all conditions of operation. Theytemperature responsive resistor 18. has a resistance of 20 ohms whencold, that is, in the absence of flamev at the pilot 20, and aresistance of 9Q Ohms when hot, that is, in the presence of flame at thepilot 2,0. The hot condition of resistor 10 creates a balance betweenheaters 6 and 12 because they both consume the same wattage. This isfurther explained by comparing the wattage in the components 6, 10 and12 under an applied voltage of 19 volts and a unity power factor, forexample. At this voltage, and with resistor It) in the hot condition,

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the heater 6 will consume approximately 1 watt, the heater 12 willconsume approximately 1 watt, and the resistor 10 will consumeapproximately 1 watt. The same wattage in heaters 6 and 12 create abalanced condition in switch 14. At the same voltage and with resistor10 in the cold condition, the heater 6 will still consume approximately1 watt, but the resistor 10 will now consume approximately .6 watt andthe heater 12 will now consume approximately 2.685 watts. Thus theheaters 6 and 12 consume different wattages and are in an unbalancedstate in switch 14, when the resistor 10 is cold. It is to be noted thatthe cold or unbalanced condition referred to is dependent upon thecondition of resistor 10 and its relation to pilot 20 and not due tolack of current flow. The current flowing through resistor 10 produces anegligible amount of heat, not enough to keep it hot when the flame isout.

Operation With no current flow from the secondary of transformer 2 theswitch 14 is in the position shown in Figs. 2, 3, and 4. Note, thatcontacts 3S44 are in engagement and contacts 4856 are out of engagement,due to an adjustment of screw 35. With the application of 19 volts fromthe secondary of transformer 2 and assuming a unity power factor in thesystem the heaters 6 and 12 and resistor 10 consume unequal wattage asin the cold or unbalanced condition. This means that heater 12,consuming 2.685 watts will provide more heat to bimetal 40, than willheater 6, consuming 1 watt, provide to bimetal 36. Thus, the bimetal 40will flex a greater distance upwardly (Fig. 3), separating contact 44from contact 33 and carrying contact 48 into engagement with contact 56,and carrying the contacts 44 and spring 41 upwardly. The bimetal 36having less heat supplied by heater 6 will follow bimetal 40 a certaindistance but not sufliciently to maintain contact 38 in engagement withcontact 44. Therefore the series switch circuit remains broken duringthe cold condition of resistor 10 which causes an unbalance betweenheaters 6 and 12.

Upon ignition of pilot 20, manually or by other suitable means, theresistor 10, being exposed to the pilot flame, heats up, changing itsresistance from 20 ohms to ohms. This changes the current drawn in thesecond parallel circuit, which causes heater 12 to decrease its wattageconsumption from 2.685 watts to 1 watt. The heater 6 is unaffectedbecause its wattage consumption remains 1 watt with the same appliedvoltage. The equal amounts of heat transmitted by heaters 6 and 12 willaffect the corresponding bimetals 36 and 40 similarly. Both bimetals 36and 40 will flex upwardly the same distance, carrying contact 38 alreadyin engagement with Contact 44 upwardly the same distance as contact 48.The bimetal 59 is unaffected by the heaters and responds only to ambientChanges. It is to be noted that bimetal 50 is constructed of bimetallicmaterial only for ambient compensation. Thus, the bimetal 50 remainsstationary and the contact 48 engages contact 56 to complete the circuitthrough pilot switch 14. Continued travel of the bimetals 36 and 46after the contacts have engaged, is provided for by the strain reliefsprings 41 and 52 which prevent distortion of the components.

Upon a demand for heat, with the closing of the contacts of thermostat16, the solenoid coil of the gas valve 18 is energized. This results infuel flowing to the main burner where it is ignited by the pilot flame.The heating system is now in operation and subsequent de-energizationand energization of the valve 18 is under the control of thermostat 16for as long a period as the pilot remains ignited. In the event that forsome reason the pilot flame is extinguished, the resistor 10 will. cool,reducing its resistance from 90 ohms back to 20 ohms. This again createsan unbalance between heaters 6 and 12. With the heater 12 providing alarger amount of heat to the bimetal 40, the contacts 44 and 38. willeparate, opening the switch circuit. Therefore, no operation of thesystem is possible without a successful and uninterrupted pilot flame.Any interruption of the pilot flame, due to clogging, wind blasts, etc.,will result in the cooling of resistor causing the unbalance andsubsequent lockout of switch 14. This feature is obtainable at allperiods of the operating cycle.

In cases of component failures the following actions occur. If heater 12burns out, the sole heating of heater 6 causes bimetal 36 to flexupwardly while bimetal 40 re mains stationary and contact 48 remains outof engagement with contact 56. The strain relief spring 41 permits thecontact 38 to push the contact 44 through the aperture 46 in bimetal 40without affecting the action of bimetal 40. The bimetal 40 remainsinactive due to the failure of heater 12.

In the event that heater 6 burns out, the bimetal 36 remains inactiveand the bimetal 40 flexes upwardly carrying the contact 44 out ofengagement with the contact 38. Even though subsequently contact 48engages contact 56, the series switch remains in the open circuitposition.

In the event resistor 10 burns out, it creates an open circuit throughheater 12 and the system reacts in the same manner as when heater 12burns out.

In the event both heaters 6 and 12 burn out, the switch remains in theposition shown in Fig. 3.

Note, that the system is substantially independent of voltagefluctuations because of the proportional change in the parallelcircuits.

As previously mentioned, a thermistor or a resistor having a negativetemperature coeflicient of resistance may be used in place of theresistor 10. In this event the unbalance is created by selecting the hotvalue of resistance of the thermistor at 90 ohms or equal to theresistance of heater 12 and the cold value of resistance of thethermistor at some high resistance, such as 1000 ohms, thereby causingthe heating effect or heat output of heater 12 to be negligible. Thenthe balanced condition after successful ignition of pilot will result inthe closing of switch 14 in the same manner as with the use of resistor10 and the unbalanced condition after a flame failure at pilot 20 willresult in movement of bimetal 40 and contact 48 downwardly, separatingcontact 48 from contact 56, while contact 38 remains in engagement withcontact 44, this latter condition being due to the negligible heatoutput of heater 12 and the continued heating of heater 6.

In the event that an increase in ambient temperature causes the bimetalblades 36 and 40 to flex upwardly, a corresponding deflection by thebimetal 50 retains the contact 56 out of engagement with the contact 48.Obviously, Where conditions warrant it, the substitution of a meal stripfor the bimetal 50 is possible without departing from the scope of theinvention. This would not provide ambient compensation but would befeasible in an application where the ambient temperatures remain stable.

This invention is subject to these and other modifications but it isintended to be limited only by the scope of the appended claims.

What is claimed is:

1. A safety control for a fuel burner of the type which breaks a controlcircuit upon the cessation of combustion at the burner comprising, asource of voltage, electrically energized fuel control means controllingthe flow of fuel to the burner, a first branch circuit connected acrosssaid voltage source and including a first electrical heater, a

second parallel branch circuit connected across said source andincluding a second electrical heater, a third parallel branch circuitconnected across said source and including in series said fuel controlmeans and a safety switch means, first and second thermally responsivemembers moveable in unison from cold to normal hot positions in responseto the application of heat thereto and mounted in thermally conductiverelation to said first and second heaters respectively, said membersbeing operatively connected to said safety switch means to close saidsafety switch means only upon movement of both said members to theirrespective normal hot positions and to open said switch means bymovement of said second member beyond its normal hot position, atemperature responsive resistance adapted to be heated by combustion atsaid burner and connected in said second branch circuit, the thermaloutput of said first and second heaters being balanced to move both saidmembers to their normal hot positions when the thermally responsiveresistance is heated by combustion at said burner, the change inresistance of the thermally responsive resistance upon the cessation ofcombustion increasing the heat output of said second heater and therebymoving said second member beyond its normal hot position to open saidsafety switch means.

2. A gas pilot safety control of the type which breaks a control circuitupon the extinction of the pilot flame comprising, a source of voltage,electrically energized means for controlling the flow of gas to a mainburner, a first branch circuit connected across said voltage source andincluding a first electrical heater, a second parallel branch circuitconnected across said source and including a second electrical heater, athird parallel branch circuit connected across said source including inseries said electrically energized means and a safety switch means,first and second bimetal members moveable in unison from cold to normalhot positions in response to the application of heat thereto and mountedin thermally conductive relation to said first and second heatersrespectively, said bimetal members being operatively connected to saidsafety switch means to close said safety switch means only upon movementof both said members to thier respective normal hot positions and toopen said switch means by movement of said second member beyond itsnormal hot position, a thermally responsive resistance having arelatively high positive temperature coeflicient of resistance adaptedto be heated by the pilot flame and connected in said second branchcircuit, the thermal output of said first and second heaters beingbalanced to move both said members to their normal hot positions whensaid resistance is heated by the pilot flame, the change in resistanceof said thermally responsive resistance upon extinction of the pilotflame increasing the heat output of said second heater and therebymoving said second member beyond its normal hot position to open saidsafety switch means.

References Cited in the file of this patent UNITED STATES PATENTS2,272,977 Slocum Feb. 10, 1942 2,287,248 Holmes June 23, 1942 2,338,786Sparrow Jan. 11, 1944 2,380,640 Eskin July 31, 1945 2,403,411 StrobelJuly 2, 1946 2,599,473 Miller Jan. 3, 1952

